Hydrogen has many industrial uses, for example production of cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals; sulfur removal from fuels during oil-refining processes; hydrogenation of oils to produce fats; etc. Hydrogen can be produced by several processes, but industrially hydrogen is often produced by removal of hydrogen from hydrocarbons, for example by steam reforming of methane. However, such processes produce waste gases (e.g., sour gas) that contain hydrogen bound in various compounds, for example ammonia. Rather than disposing of the waste gases, it would be economically beneficial to recover hydrogen from the waste gases.
Sour gases containing sulfur can generally be directly fed into a Claus plant for conversion into elemental sulfur, or can be processed to enrich the gas in hydrogen sulfide, if necessary, prior to feeding the Claus plant. While the Claus reaction (converting hydrogen sulfide to elemental sulfur and water in the presence of oxygen) is conceptually simple and relatively easy to implement, significant limitations are present where the feed gas has relatively high concentrations of ammonia. For example, ammonia can be found in relatively high concentrations in sour water stripper gases, coke vessel off gases, off gases from acid gas removal units, low-temperature gasification off gases, and/or hydroprocessing off gases.
Regardless of the particular source, Claus plants require significant modifications to cope with high ammonia concentrations (e.g., require a modified Claus burner and/or use of oxygen enrichment for thermal decomposition). Some Claus processes could employ dedicated upstream systems, or combust ammonia and hydrogen sulfide in a single Claus thermal stage. However, most of these systems and methods tend to require significant modification of existing Claus plants and thus are not economically attractive.
Some methods contact ammonia with a concentrated acid in an absorption step to produce an ammonium salt solution that is then transferred to a stripper operating at a relatively low temperature to remove hydrogen sulfide therefrom, before the stripped solution is contacted with a base to obtain a corresponding crystalline ammonium salt. The remaining liquid can then be recycled to the absorption step. While such methods can be fairly effective in separating ammonia from hydrogen sulfide, numerous problems arise. For example, due to the concentrated acid solution, precipitation of corresponding ammonium salts may occur in the absorption step, especially where the ammonia concentration is relatively high. Also, due to the stripping step, the separated hydrogen sulfide may be released in a more diluted form that may necessitate a downstream concentration step.
Other methods sequentially isolate ammonia and hydrogen sulfide (and other impurities) in a process where ammonia is removed at low temperatures and elevated pressure using an aqueous acidic wash liquid, while hydrogen sulfide is removed in a later step via an alkaline washing liquid. Such methods not only require substantial equipment, but also regeneration of multiple solvents, adding significant costs. Moreover, such methods are limited to a maximum concentration of ammonia of 0.6 vol. %. Thus, there is an ongoing need for the development of methods for processing sour gases, for example sour gases that feed into sulfur recovery plants, by recovering important chemicals, such as hydrogen.